PPA fixes

This PR:

- fixes already added PPA detection (path was wrong for the `sources.d`)
- add normalization of the repo for the PPA check (Ubuntu >= 12.04 replaces dots with underscores)
- adds `auto_accept=True` to pass to `add-apt-repository` if on Ubuntu >= 12.04
- adds the possibility to specify a keyserver

In details:

Normalization now works on 10.04 **and** 12.04 so I closed #70 and #71.

As for 10.04, not sure how we want to handle the confirmation, as there is no option like you mentioned before.

For now, if on ubuntu >= 12.04, it confirms by default.

For the key server, I made sure that if they tried to pass multiple key servers, to just grab the first one (instead of erroring), but it may not be needed.

Feedback welcome, thank you !